1. Field of the Invention
The present invention relates to a method and a control device for controlling a quantity control valve of a high-pressure pump.
2. Description of the Related Art
Published German patent application document DE 101 48 218 A1 describes a method for operating a fuel injection system while utilizing a quantity control valve. The known quantity control valve is implemented as a solenoid valve operated electromagnetically by a solenoid coil, having an armature and associated travel-limiting stops. Such quantity control valves, which are closed in the currentless state of the solenoid coil, are known from the market. In this case, for opening the quantity control valve, the solenoid coil is controlled using a constant voltage or a clocked voltage (pulse-width modulation—“PWM”), whereby the current in the solenoid coil rises in characteristic fashion. After the voltage is switched off, the current drops off, in turn, in characteristic fashion, whereby the quantity control valve closes. Solenoid valves, which are open in the energized state of the coil, are also known. In these solenoid valves, there is a corresponding procedure, the solenoid valve opening when the voltage is switched off and the current drops in a characteristic manner.
In the valve that is closed in the currentless state shown in published German patent application document DE 101 48 218 A1, in order to prevent the armature from hitting the stop at full speed during the opening motion of the quantity control valve, which could result in a marked noise development, the electromagnetic actuating device is once more supplied with current, in a pulse-like manner, shortly before then end of the opening motion. A braking force is exerted on the armature by this current pulse, still before it contacts the stop. The braking force reduces the speed, whereby the noise of striking the stop is diminished.
In modern direct injection engine systems having demand-controlled fuel delivery, a high-pressure pump is used for generating the necessary fuel pressure. For this purpose, the high-pressure pump is operated in a quantity-controlled manner, the delivery quantity of the pump being able to be set from 0 to 100% by a quantity control valve. Controlling this quantity control valve is of particular importance since the switching process of the quantity control valve must occur in a very short time and in spite of high magnetic forces due to the high rotational speed and the associated high control frequency, without the lift-to-lift fluctuations and therefore the delivery quantity fluctuations becoming too great. This would result in a lack of rail pressure quality. On the other hand, at low engine speeds, very high demands are made on noise development of the high-pressure pump. For this reason, numerous control concepts have already been developed to reduce the impact dynamics, and thus to reduce the acoustical level. In the process, both the starting motion as well as the decreasing motion of the switching magnet are slowed down.
A control concept for a quantity control valve is described in published German patent application document DE 10 2009 046 825 A1, which is also called a “current soft stop” (CSS). Normally, the quantity control valve is held closed beyond top dead center by the pressure in the delivery chamber of the high-pressure pump. If the delivery chamber pressure drops off, the quantity control valve falls back into the original currentless open position, driven by spring force and non-braked. In the CSS method, the quantity control valve is supplied beyond the upper dead center with a holding current, so that the quantity control valve does not yet drop off immediately. Only after the pressure reduction in the delivery chamber is the current reduced in characteristic fashion, so that the quantity control valve drops off during this low current supply, and falls back into the currentless open position. Because of the counter-induction and the current running through the quantity control valve, the motion is braked in the process, and the impact at the stop takes place substantially less rapidly, and thus more quietly. The holding current should be known as accurately as possible, so that currents for holding and for the onset of motion may be set as precisely as possible. The application of current must be terminated again before the following lower dead center, so that the next delivery process is not disturbed.
It is a problematic fact that, if the current is too low, the CSS method yields only slight acoustical improvements, while currents that are too high cannot effect any improvement, or may even cause an acoustical deterioration and a rail pressure rise. This is due to the fact that, at currents that are too high, the quantity control valve remains closed and does not open.
Since the manufacturing tolerance has to be taken into account in the data input for the current, the effect would be limited under these conditions, since usually the data input is made in such a way that the quantity control valve opens with certainty. That is to say that a current would be selected that is rather too low. In the case of this low current, only a slight improvement is then possibly obtained.